Pitchers aren’t often groomed as closers, they usually fall into the role (if they’re good enough) after struggling as a starter. Edwin Diaz had nothing but success as a starter in the Minor Leagues, but the Seattle Mariners saw Diaz’s 98-mph fastball and nasty slider and saw their next star closer.

Last season, Diaz was in the middle of another strong season as a starter in Double-A and it looked like the next step might be Triple-A. But after six starts, the organization moved Diaz to the bullpen to be groomed as their next closer. The grooming didn’t take long, as Diaz rifled off 16 strikeouts in 11 2/3 scoreless innings of relief before bypassing Triple-A and getting the call straight up to the Mariners.

After two months, Diaz had earned 13 holds, owned a tidy 1.80 ERA and struck out a ridiculous 49 batters in 25 innings. On Aug. 2, Diaz took over as the closer, earned his first save and never looked back. He finished the year on a bit of a rocky note with a few bad outings that raised his ERA. Nonetheless, he showed more than enough to be considered one of the most exciting young closers in all of baseball entering this season.

Diaz’s rookie year was one of the best for a reliever in recent memory. He led all pitchers with an almost unimaginable 15.33 K/9 and led all relievers with a 1.88 xFIP, 40.6 K% and 18.5 SwStr% (swinging-strike percentage).

While his fastball is more than impressive, it’s Diaz’s slider that gives hitters fits. He threw it 32 percent of the time and it clocked in on average just under 87 mph. According to PITCHf/x pitch values, his slider earned a rating of 10.7, the second highest in baseball. The slider itself yielded a 33.5 SwStr% and a 29.4% ground ball rate; according to Eno Sarris’ blog on Fangraphs, this results in a 6.6 SumZ which rates it as the 14th best pitch and third best slider in all of baseball last year.

Diaz has already pitched in some ramped up game action this spring, serving as the closer for Puerto Rico in the World Baseball Classic. He fared well, striking out nine in 5 1/3 innings while giving up two runs on two hits. In his first four Spring Training appearances, he threw 3 2/3 scoreless innings, striking out six.

Many 23 year olds entering their first full season in the bigs aren’t projected to have as much success as systems like Steamer and Depth Charts are giving Diaz. He is projected by both to save 34-35 games with a sub 2.80 ERA while fanning more than 11 batters per nine innings. Given his success in the Minors and easy transition to the bigs, there’s really little reason to believe Diaz can’t repeat, let alone improve on his impressive rookie year.

Rich Arleo is a freelance sports writer and editor who covers Major League Baseball and fantasy sports. You can follow him on Twitter, @Rarleo.